Extruders are used for a number of purposes. One such purpose is to push an extrudate, such as pet food, through an extrusion die, where the pet food extrudate is cut off into pellets of desired shape and size. Typically, the flited extrusion screws are made of steel and the screw rotates in a housing or barrel. The screw shaft is supported only at the driven end (where it is coupled to a motor or transmission). The screw and its flites are centered in the housing by interaction between the screw flites, the extrudate, and the inner diameter of the housing (the theory being that the screw is supported by the extrudate and does not contact the interior of the barrel). Because steel screws are tough and fracture-resistant, this arrangement is satisfactory in most cases.
In the pet food application, among others, the extrudate is highly abrasive. Accordingly the screws and screw flites are covered with wear-resistant material, such as tungsten carbide, in the form of tiles or welded-on hardfacing. One such screw is disclosed in commonly assigned U.S. Pat. No. 7,513,676 to Williams et al., which is incorporated herein by reference. The wear- or abrasion-resistant material on such screws is hard but brittle, and can be damaged or destroyed by contact between the screw flites and the inner diameter or wall of the housing. This contact is not prevented by simply “floating” the screw on the extrudate. Accordingly, a need exists for a mechanism or apparatus for stabilizing a wear-resistant extrusion screw within the housing in which it rotates.